who (to describe people – subject)
The woman who works in the bank is my neighbor.
whom (to describe people – object)
My cousins, one of whom is a doctor, live in England.
whose (to describe possession)
The man whose car was stolen went to the police station.
that (to describe things – defining relative clauses)
I’m selling the computer that I bought in the U.S.
which (to describe things – non-defining relative clauses)
I’m selling this computer, which has a 250-GB hard drive, for $500.
